This invention relates to the formulation and configuration of a heat reactive resin system at temperatures below the melting temperature (Tm) of the base resin in the system. More particularly the invention relates to the preparation of coating powders, the application of coatings and the formation of shapes from fluid heat reactive resin systems and curing them at low temperatures.
1. Definitions
As used in the specification and the attached claims, the following terms are defined as follows.
a. “Base resin” or the unmodified term “resin,” means a neat, heat reactive resin to which no curing agents have been added.
b. “Resin mixture” means a simple mixture of resins and other ingredients such as curing agents, pigments, additives and the like before dispersing the other ingredients in resins.
b. “Normally solid” is used in the context of ambient room conditions.
c. “Heat reactive resin systems” means resins in which curing agents, catalysts, pigments, additives, fillers and the like have been dispersed.
d. “Slowly,” when used to modify how the pressure in a pressure vessel is relieved, indicates that the pressure is relieved in a controlled manner that avoids a significant formation of a foam.
e. “Transient processing time” is the elapsed time that a heat reactive resin system discharged from a pressure vessel remains fluid enough to be configured.
f. “Low temperature curing” means the cure of a heat reactive resin system is carried out at a temperature below about 140° C.
g. “Supercritical range” means the conditions of temperatures and pressures at which liquefiable gases are approaching, at or somewhat above their supercritical point. Note that this is consistent with prior usage as illustrated, for example, with the definition of “supercritical fluid” as used in U.S. Pat. No. 5,027,742 at line 23 of column 4, the teachings of which patent are incorporated herein by reference as they relate to liquefiable gases and supercritical fluids.
2. Prior Art Discussion
Commercially important materials such as paints, adhesives, molding compounds, coating powders, toners, pharmaceuticals are commonly prepared from polymers in which curing agents, pigments, fillers and the like are dispersed. The dispersions may prepared in liquid dispersing equipment such as ball mills, media or bead mills, and in high shear mixers such as a Cowles dissolver, colloid mills and the like while the polymers are dissolved in a solvent. Dispersion processes are normally carried out at atmospheric temperatures and pressures.
If the solid ingredients are dispersed in polymers such as rubber, plastics or resins, the polymers are processed in a plastic or molten state. Typical apparatuses for carrying out such dispersions are Banbury Mixers, 2-roll mills and extruders of all types. In these devices the polymer is heated above its softening temperature by external heating, by functional heating of the plastic mass, or by the dissipation of mechanical action (work.) Even low melting resins such as those used in the preparation of thermosetting coating powders or electrostatic toners, require processing temperatures of at least about 100° C. and most usually significantly higher temperatures. If the curative in the thermosetting resin systems are designed for low temperature curing, they are, at least, partially reactive at the normal extrusion temperatures e.g. 100° C. or more.
Thermosetting polymer mixtures are conventionally prepared by thoroughly dry mixing resinous binders with components such as curing agents, additives, pigments, fillers, catalysts, etc. and then dispersing the ingredients above the melting point of the resinous binders. This is commonly referred to as “melt mixing.” A description and examples of melt mixing can be found in the Kirk-Othmer Encyclopedia of Chemical Technology, Volume 6, starting on page 635 (1993).
In the preparation of heat reactive thermosetting resin systems such as are useful as coating powders, adhesives and the like, it is often desirable to formulate reactive resin systems that cure at relatively low temperatures or rapidly at higher temperatures. Care must be taken when the reactive resins mixtures are dispersed above their melting points to avoid premature reactions that yield gelled or partially gelled products. Even if premature reactions do not result in partially gelled products, the curing reaction, once started, may continue at ambient temperatures resulting in cross linking reactions and reduced flow of the dispersions when used in coating processes. Eventually, the reaction will proceed until the flow is impaired to the extent that a continuous film can no longer be formed. The ability of a thermosetting composition to maintain good application characteristics over a long period of storage is referred to as “storage stability” or sometimes “shelf life.”
There are many applications where powder coatings cannot be used because of the high temperatures required to melt and cure the coating in a reasonable time, e.g. 30 minutes or so. Most of these applications involve coating temperature sensitive substrates such as plastics, various types of wood or engineered wood products such as particle board, oriented strand board (OSB) and medium density fiberboard (MDF) and composite assemblies containing rubber, plastics electrical insulation, etc. In order to utilize reactive curing agents necessary for low temperature curing thermosetting coatings, it is necessary to process them at temperatures below which significant reaction between the resin and the curative occurs.
Melt mixing processes are made more difficult if the resins have significantly differing melting points and variations in melt viscosity. Melt mixing temperatures must be sufficiently high to melt the resin and form a useful and processable melt viscosity but, on the other hand, the temperatures must not be so high as to lower the viscosity of additives to a point at which good dispersions can not be achieved. That is to say that melt mixing is most effective when the viscosities of the melted materials are of a similar magnitude. For example, when melt mixing or dispersing crystalline or partially crystalline resins, additives, catalysts, waxes, etc. with amorphous polymers, the lack of homogeneity of the melt viscosities can result in micro defects in coatings applied from the dispersion. Micro defects lead to haze and loss of clarity. This can diminish the utility of coating materials in demanding applications such as automotive finishes in which a high level of gloss and distinctiveness of image are highly desired.
Gaseous fluids are commonly used in extraction and impregnation processes. Exemplary of this technology are U.S. Pat. Nos. 3,969,196, 4,061,566, 4,308,200 and others.
In U.S. Pat. No. 4,598,006, thermoplastic polymers are impregnated with fragrances, pest control agents and pharmaceuticals dissolved in a volatile swelling agent for the polymer, where the volatile swelling agent is a gas maintained above or near supercritical conditions. When the pressure or temperature is reduced, the gas diffuses out of the thermoplastic polymer but the impregnated material remains in the polymer. The reduction in temperature or pressure is carried out carefully so the physical appearance of the polymer is not altered.
U.S. Pat. No. 4,820,752 discloses a method of infusing an additive into a polymer using a compressed fluid which is normally a gas at room temperature. The fluid may be in a liquid or gaseous state if the operating environment at which the process is being carried out is below or equal to the critical temperature of the fluid. If the operating environment is above the critical temperature of the fluid, the process must be carried out in the liquid state. The fluid and the additive are chosen so that the additive has a degree of solubility in the polymer into which it is to be infused and so that the solution of fluid and additive has a degree of solubility in the polymer and is capable of swelling the polymer. The polymer is swollen at least 2% by volume and preferably 5% by volume by the compressed normally gaseous fluid used. Carbon dioxide is the preferred fluid.
In both of these patents, the form and appearance of the polymer is not significantly altered but the additive must be soluble in the supercritical fluid. If the additive is not soluble in the supercritical fluid, it cannot be imbibed by, or carried into, the structure of the polymer. For a more complete description of the solvation of resins in supercritical fluids, see U.S. Pat. No. 4,734,227 whose teachings are incorporated herein by reference.
U.S. Pat. No. 5,708,039 describes a method for producing a coating powder by dissolving the ingredients in a combination of an active solvent and supercritical carbon dioxide. The solution is sprayed from the supercritical solution which reportedly forms generally spherical particles. The remaining solvent is subsequently removed by evaporation under vacuum.
The use of liquefied gasses in the supercritical state for processing resin mixtures is described in U.S. Pat. No. 5,399,597. Thermosetting resin mixtures are dispersed in a supercritical gas, preferably supercritical carbon dioxide, in a first pressure vessel. The dispersion process is carried out by agitating the resin mixture with the supercritical carbon dioxide until the desired degree of dispersion is obtained. The dispersion of thermosetting resin and supercritical carbon dioxide is then atomized through hydraulic spray nozzles into a second vessel maintained at a lower pressure. A conglomerate of flake-type and rounded particles is reported to be formed. Spraying the resin mixture and carbon dioxide directly from the supercritical state to form powder particles is a critical part of the patented process and must be observed in all cases. It is taught that none of the ingredients in the resin mixture should be soluble in the supercritical carbon dioxide to avoid volatilization (separation) when the resin mixture is sprayed from the supercritical state.
A process is disclosed in U.S. Pat. No. 5,975,874 for compounding thermosetting coating powders in an extruder. A supercritical fluid is utilized in the extruder to reduce the viscosity of a coating powder precursor although the patent does not discuss how the supercritical fluid is contained within the extruder. It is taught that coating powders are made from the extruded materials.
U.S. Pat. No. 5,981,696 is of interest for its disclosure of dissolving base resins and a plurality of hardeners in an inert low molecular weight compound while the compound is maintained above its critical temperature and pressure. The resins and hardeners are processed within a pressure vessel and, it is said, coating powders may be formed by spraying the solution into a region of lower pressure. Alternatively, the patent teaches that the pressure may be relieved in a time-dependent manner. In this later case, the reactor is described as containing a solid foam after a normal pressure is reached. The foam is described as consisting of individual particles which adhere to each other but which can be separated into individual particles.